This invention relates to a method for the vacuum deposition of thin coatings on substrates which are heated during the evaporation to more than 500.degree. C. by electron beam heating, the substrates being held above an evaporating crucible filled with a bath of the material being deposited which is likewise heated by electron beams.
From a publication by Chromalloy Corporation entitled "High Temperature Resistant Coatings for Superalloy" Seelig et al., it is known to deposit surface coatings of highly resistant alloys from vapors onto gas turbine blades for the purpose of increasing the load capacity of the blades. Special attention must be given in this case to a reliable bonding of the vacuum deposited coating or coatings to the substrate. The strength of adhesion can be substantially improved, for example, by intermetallic difussion, it being necessary for this purpose to keep the substrates, such as turbine blades and the like, at a high temperature level in the range of, for example, of 900.degree. C. to 1000.degree. C. during the vacuum deposition.
It is known to provide resistance radiant heating bodies or special electron guns for the heating of the substrate. It is likewise known to deflect periodically the electron beam used for the heating of the material being vaporized such that it will impinge alternately upon the material being vaporized and the substrate. In this case, however, no electron beam heating of the bath takes place while the electron beam is dwelling upon the substrate. The electrons which are necessarily reflected and not utilized for heating are in any case lost insofar as heating purposes are concerned, since in the known process no attention has been paid to the arrangement among the electron beam source, the bath and the substrate.
To a small degree, the substrates are heated by thermal radiation of the bath and by the heat of condensation of the deposited coating material. However, at best, only about 10 to 15% of the overall heat requirement of the substrates can be supplied in this manner.